[derefr]

"Happiness" is a vague emotion, like love, not an objective, short-term, measurable one, like calm, contentment, or excitation. However, all states of happiness usually share in common a factor of endorphin release.

Endorphins are basically one of two basic mechanisms for "fitting" data in the brain-as-neural-net, with the other being pain: pain is the reinforcement mechanism that trains your neurons to not do that again, while endorphins tell your brain to do please do that again. People are "happy" (have endorphins) when they're doing things that their body encourages them to do. To be happy is simply to be one with your body's material desires (though note that your brain is part of your body, and it has material desires of its own, for things like friendship.)